A handover is a process of transferring an ongoing call or data session from one cell to another. Conventionally, a wireless transmit/receive unit (WTRU) assisted network controlled handover is implemented such that a WTRU measures a signal strength of neighboring cells and a serving cell and sends a measurement report to the network. The network then determines whether to perform the handover to another cell, (i.e., a target cell).
FIG. 1 is a signaling diagram of a handover process 100 in the third generation partnership project (3GPP) long term evolution (LTE) network (infra-mobility management entity (MME)/serving gateway handover procedure). The source evolved Node-B (eNB) configures the WTRU measurement procedures according to the area restriction information (step 102). The WTRU sends a measurement report in accordance with the rules set by system information, specification, etc. (step 104). The source eNB makes a handover decision based on the measurement report (step 106). The source eNB issues a handover request message to a target eNB passing necessary information to prepare the handover at the target eNB (step 108). Admission control may be performed by the target eNB (step 110). Once the target eNB decides to admit the WTRU, the target eNB prepares handover with L1/L2 and sends a handover request acknowledgement message to the source eNB (step 112). The handover request acknowledgement message includes a transparent container to be sent to the WTRU as a radio resource control (RRC) message. The source eNB sends a handover command to the WTRU (step 114).
Upon receipt of the handover command, the WTRU detaches from the source cell and synchronizes to the target cell and accesses the target cell via an RACH following a contention-free procedure if a dedicated RACH preamble was indicated in the handover command or following a contention-based RACH procedure if no dedicated RACH preamble was indicated in the handover command (step 116). The target eNB sends a random access response with uplink allocation and timing advance value for the WTRU (step 118). The WTRU then sends a handover complete message to the target eNB (step 120). A normal operation starts between the WTRU and the target eNB thereafter.
A WTRU needs to know a system frame number (SFN) after handover for its normal operation in the target cell. Especially, both discontinuous reception (DRX) and reception of dynamic broadcast channel (D-BCH) require the WTRU to have the knowledge of SFN. Therefore, an efficient method would be desirable to handle SFN during and after handover.